The heddles of a loom must be shifted between the low-shed and high-shed positions at particular times in the loom cycle, normally after the pick is made and the weft is beaten in, to create a standard patterned dobby or jacquard weave. After each pick some of the heddles must be moved from the low-shed to the high-shed position, some in the reverse direction, while others must remain in whichever of the positions they are already in. The selection is made by a reader which generates for each pick an output for each heddle. Each of these outputs is formed by a respective control rod which is movable a very short distance between a position corresponding to the low-shed position and one offset thereform and corresponding to the high-shed position.
Due to the high speed of modern looms it is impossible for the reader to move the heddle control rods at the exact instants when switchover between high- and low-shed positions is possible. Thus displacement of a control rod into the position for a given pick takes place some time during the preceding pick. For example, when a given heddle is in the low-shed position during a pick, its control rod can be displaced by the reader into the high-shed position. Mechanism then switches over the heddle later at the proper instant for the next pick.
It is standard practice to control the heddles by respective eccentric cams carried on a common shaft that rotates continuously or discontinuously. In one angular position of the cam the respective heddle is in the low-shed position and in another angular position, normally offset by 180.degree. from the low-shed position, the heddle is set in the high-shed position, with switchover only possible in these two angularly offset cam positions. This switchover is accomplished by providing between the main drive shaft and each cam a respective latch which can couple and decouple the respective cam from the shaft carrying it. This latch is in turn controlled by the respective control rod. If the heddle is in, say, the low-shed position and for the next pick it is to be in the high-shed position, at the end of the pick its latch will couple it to the shaft. If a heddle is to stay in the same position for the next pick, its latch is left uncoupled.
German patent document 1,410,729 describes such a system wherein a radially displaceable wedge can press latching balls seated in the cam against the drive shaft, thereby coupling the cam to the shaft. Similarly U.S. Pat. 3,468,347 of Fumat shows a system having a latch pawl pivoted for movement about an axis parallel to the drive shaft on the cam, and having a radially displaceable tooth engageable in radially outwardly open notches of the drive shaft. In U.S. Pat. No. 3,804,128 of Amigues another such arrangement is used which has a radially displaceable latch member or cotter which is urged radially inwardly by springs toward a position engaged in a notch of the shaft, but which can be forced radially outardly from this position by a lever operated by the respective control rod.
These systems all have the considerable disadvantage that the latching mechanisms do not work well at high speed. The main problem is that the high centrifugal forces brought into play at high speed occasionally inhibit the desired motion of the latch mechanisms, which all rely on radially dispaceable elements. Thus if a given part is to be moved radially inwardly, at high speed it must be moved against considerably outwardly effective centrifugal force. Indeed in very high-speed operations some of the latch members can even be thrown centrifugally into the outer positions to put the respective heddles in the wrong position and ruin the goods being produced.